1. Field of the Invention
The present invention relates to a multi-layer capacitor or other electronic device, a dielectric ceramic composition suitable for use as the dielectric layer of the electronic device, and a method for producing the same.
2. Description of the Related Art
A multi-layer ceramic capacitor is being broadly used as a compact, large capacity, high reliability electronic device. The number used in each piece of electrical equipment and electronic equipment has also become larger. In recent years, along with the increasing miniaturization and improved performance of equipment, there have been increasingly stronger demands for further reductions in size, increases in capacity, reductions in price, and improvements in reliability in multi-layer ceramic capacitors.
One of the key technologies for reducing price is the use of the relatively inexpensive Ni or Ni alloys instead of use of the high priced Pd and Pd alloys for the internal electrodes. Further, the key technologies for the reduction of size and the increase in capacity are the reduction in thickness of the dielectric layer and the use of multiple layers.
If the thickness of the dielectric layer is reduced, the electric field intensity acting on the dielectric layer when a DC voltage is applied becomes larger. Along with this, the phenomenon of the reduction in the electrostatic capacity and the insulation resistance becomes remarkablexe2x80x94in particular in high dielectric constant dielectric ceramic compositions. From previous reports on the DC voltage dependency of the electrostatic capacity, that is, the DC bias, it is widely known to add to a main component of barium titanate, subcomponents such as Bi2O3, TiO2, SnO2, ZrO2, and other compounds and rare earth elements. When a dielectric ceramic composition containing these compounds as subcomponents is used as the dielectric layers in a multilayer capacitor, however, the Pd of the internal electrode layer and the subcomponent compounds (for example, Bi2O3) react and the characteristics of the capacitor become insufficient. Therefore, it is necessary to use Pt or Au, which are both more expensive than Pd, for the internal electrode layers.
Further, as a dielectric ceramic composition not containing a compound such as Bi2O3, there are known dielectric ceramic compositions comprising a main component of barium titanate and subcomponents of Nb2O5, Co2O3, Nd2O5, MnO2, and SiO2 (Japanese Unexamined Patent Publication (Kokai) No. 6-203630). A multilayer capacitor using this dielectric ceramic composition as dielectric layers and using 30% Ag-70% Pd alloy as the internal electrodes has a temperature change of the electrostatic capacity TCC which satisfies the X7R characteristic and has a rate of change of the electrostatic capacity xcex94C/C of within xe2x88x9230 percent when applying a DC bias field of 2 kV/mm. It is difficult to apply this dielectric ceramic composition, however, to a multi-layer capacitor using Ni as internal electrode layers.
Note that, as shown in Japanese Unexamined Patent Publication (Kokai) No. 10-330160, to improve the insulation breakdown voltage, there is known a barium titanate-based dielectric ceramic composition wherein Mn or another additive is substantially uniformly distributed in the entire region from the grain boundary to center of the crystal grain having a core-shell structure. In such a dielectric ceramic composition, however, the dielectric constant is insufficient and the temperature change of the electrostatic capacity TCC does not always satisfy the X7R characteristic of the EIA standard.
An object of the present invention is to provide an electronic device such as a multi-layer capacitor which can satisfy both of the X7R characteristic (EIA standard) and B characteristic (EIAJ standard) of the temperature characteristic of the electrostatic capacity, has little voltage dependency of the electrostatic capacity and the insulation resistance, is superior in insulation breakdown resistance, and can use Ni or a Ni alloy as the internal electrode layer, a dielectric ceramic composition suitable for use as the dielectric layer of such an electronic device, and a method of producing the same.
The present inventores engaged in intensive studies to achieve the object and as a result discovered that a dielectric ceramic composition comprised of barium titanate and a component M as main components and having a ferroelectric phase region in which the concentration of the component M in the ferroelectric phase region changes from the outside toward the center has superior properties and thereby completed the present invention.
That is, according to the present invention, there is provided a dielectric ceramic composition comprising as main components barium titanate and a component M (wherein M is at least one type of component selected from manganese oxide, iron oxide, cobalt oxide, and nickel oxide) and having a ferroelectric phase region, wherein the concentration of the component M in the ferroelectric phase region changes from the outside toward the center thereof.
The concentration of the component M in the ferroelectric phase region is preferably higher at the outside compared with near the center of the region.
The ferroelectric phase region is preferably comprised of an outside ferroelectric phase region and an inside ferroelectric phase region and has a higher concentration of the component M in the outside ferroelectric phase region than the inside ferroelectric phase region. In this case, more preferably, the inside ferroelectric phase region does not contain almost any of the component M.
In the dielectric ceramic composition of the present invention, generally there is a diffusion phase region outside of the ferroelectric phase region.
According to the present invention, there is provided a method of producing a dielectric ceramic composition comprising the steps of calcining barium titanate (A) and an ingredient of a component M (where M is at least one type of component selected from manganese oxide, iron oxide, cobalt oxide, and nickel oxide) and firing a mixture of the compound obtained in the calcination step and other barium titanate (B).
The temperature at the calcination is preferably 1000 to 1300xc2x0 C.
The firing may be performed under a reducing atmosphere.
The molar ratio (M/A) of the component M to the pre-calcination barium titanate (A) is preferably 0.0010 to 0.0120, more preferably 0.0020 to 0.0080. Further, the molar ratio (B/A) of the later adding barium titanate (B) with respect to the pre-calcination barium titanate (A) is preferably 0.05 to 5.00, more preferably 0.10 to 1.00.
According to the present invention, there is provided an electronic device having a dielectric layer, wherein the dielectric layer is comprised of the above dielectric ceramic composition.
In the present invention, the xe2x80x9cferroelectric phase regionxe2x80x9d means the inside of a portion where a boundary is observed inside of a crystal grain when observing the microstructure of a dielectric ceramic composition by a transmission electron microscope (TEM). The ferroelectric property of barium titanate (BaTiO3) is derived from the bipolar moment arising due to the displacement of the Ti ions. When atoms other than Ti atoms solidly dissolve into the barium titanate, the dielectric constant falls, the electrostatic capacity and the insulation resistance become blunted with respect to the voltage applied, and the ferroelectric property falls.
Therefore, the inside ferroelectric phase where the concentration of the component M is small contributes to an improvement of the dielectric constant, while the outside ferroelectric phase where the concentration of the component M is high has a small ferroelectric property. The ferroelectric region in the dielectric ceramic composition of the present invention is comprised of at least these two or more ferroelectric phases. As a result, it is possible to provide a dielectric ceramic composition having a high dielectric constant, a small temperature dependency of the electrostatic capacity, and small voltage dependencies of the electrostatic capacity and insulation resistance.
Note that when the concentration of the component M is relatively low and the component is uniformly distributed in the ferroelectric phase region, the dielectric ceramic composition has a larger dielectric constant, but there is the problem that the dielectric ceramic composition has a larger voltage dependency of the dielectric constant. Further, when the concentration of the component M is relatively high and the component is uniformly distributed in the ferroelectric phase region, the dielectric ceramic composition has a smaller voltage dependency, but also a lower dielectric constant.